Polymer Ultrasound Contrast Agents for Targeted Drug and Gene Delivery

Ultrasound contrast agents (UCA) are encapsulated gas microbubbles that can respond to ultrasound by rapidly expanding and contracting (exhibiting cavitation) while also reflecting the acoustic signal. This ultrasound induced cavitation can be utilized to increase the permeability of blood vessels or to create transient pores in cell membranes. It may be possible to take advantage of these biological effects for ultrasound targeted drug and gene delivery. Microbubbles made with a poly(lactic acid) shell and loaded with Doxorubicin (Dox) were previously developed in this lab and shown to rupture into 200-400nm drug loaded polymer fragments when exposed to ultrasound. The ability of this platform to provide ultrasound-targeted drug delivery in vivo was examined with a rat hepatocellular carcinoma model and compared with free Dox. Microbubbles were able to deliver significantly more drug to the tumor compared to free Dox (2.491 ± 0.501%/g tissue, vs. 0.373 ± 0.087%/g tissue, p= 0.047). However, significantly higher drug levels were also found in the spleen and liver of animals treated with microbubbles compared to free Dox. Microbubbles were also loaded with more hydrophobic drugs including paclitaxel and docetaxel. Total payloads of 129.5 ± 1.8 and 80.8 ± 3.0μg drug/mg UCA were achieved for paclitaxel and docetaxel loaded microbubbles while maintaining the agents‟ acoustic activities. Both agents were able to provide a sustained in vitro release of drug from the shell over three weeks and the drug released from these agents was able to prevent the growth of MCF7 cancer cells in vitro. Polyethyleneimine coated polymer microbubbles were also developed for ultrasound targeted gene therapy and were capable of carrying a DNA payload of 22.8 ± 0.13μg DNA/mg UCA. Additionally, polymer microbubbles triggered with ultrasound enhanced the delivery of free plasmid DNA into the cytoplasm of MCF7 cells in vitro. Successful transfection required an acoustic pressure amplitude of 250kPa or greater and showed a strong dependence on frequency and pulse length, however these settings also resulted in significantly lower cell viability. Transfection efficiency was also affected by the calcium ion concentrations in the culture medium and stage of the cell cycle that cells were treated in.%%%%Ph.D., Biomedical Engineering  – Drexel University, 2012